Apparatus for reducing noise



June 6, 1961 G. M. LILLEY ETAL APPARATUS FOR REDUCING NOISE Filed March 28, 1956 2 Sheets-Sheet 1 muuu-lun FIG. 3

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June 6, 196,1

G. M. LILLEY ErAL 2,987,136 APPARATUS FOR REDUCING NOISE Filed March 28, 1956 2 Sheets-Sheet 2 x waarna/.5

United States Patent O i y APPARATUS FOR REDUCING NOISE Geoffrey Michael Lilley, lCranfield, Bletchley, England, and Douglas James Green, Treorchy, Rhondda, Wales, assignors to Power Jets (Research and Development) Limited, London, England, a British company Filed Mar. 28, 1956, Ser. No. 574,433 Claims priority, application Great Britain Mar. 31, 1955 3 Claims. (Cl. 181-57) 'I'he invention relates to apparatus for reducing the noise emitted by high speed uid streams discharged from nozzles and the like.

One application of the invention is to the suppression of the jet noise of jet propulsion engines, e.g. turbo jets, ram jets and rocket motors, when they are being run up on the ground. The invention may also be applied to silencing the exhaust from other prime movers, e.g. piston engines, in which the exhaust stream is not intended to give propulsive thrust.

The present invention is based on the realization that to reduce jet noise, it is desirable to reduce the jet velocity. In the applications referred to above, however, it is desirable that either no back pressure shall be exerted on the engine or that any existing back pressure -shall remain unchanged. An object of the invention is, therefore, to reduce the noise of a fluid stream being discharged from a nozzle without substantially changing the conditions of the stream at the nozzle.

The invention accordingly provides apparatus for reducing the noise emitted by a fluid stream discharged from a nozzle, comprising an open-ended divergent duct, the smaller end of the duct constituting an inlet for the stream, and a plurality of axially spaced Walls extending transversely across the duct and formed with apertures to permit the flow of the stream along the duct, the spacing of the upstream transverse Wall from the duct inlet and the spacing of each subsequent transverse wall from the transverse wall immediately upstream thereof being so chosen in relation to the pressure drop in the stream created by that transverse wall, the configuration of the duct and the velocity of the stream that the stream velocity is reduced while separation of the boundary layer from the duct wall is substantially suppressed, and the noise emitted by the stream is reduced.

According to a feature of the invention the flow area through the upstream transverse wall is notless than the ow area of the duct inlet, otherwise a back pressure may be exerted giving rise to a reduction in the rate of mass tlow through the nozzle.

According to `a further feature of therinvention, the total pressure drop created by the :transverse Walls is such that a predetermined relationship between the static pressures at the duct inlet and outlet is maintained. Thus if `the stream is to be discharged from the nozzle at ambient pressure, the total pressure drop created by the transverse walls is such that the static pressure of the stream at the duct inlet is maintained substantially equal to ambient pressure. If, therefore, the stream is the jet stream of a jet propulsion engine, there is no substantial rise of back pressure on the engine. If `the stream is discharged from a choked nozzle, i.e. one in which the static pressure of the stream discharged from the nozzle is above ambient, the total pressure drop created by the transverse walls is such that the static pressure at the duct inlet is maintained at a value substantially equal to that of the stream when discharged freely from the nozzle, i.e. the existing differential between the iluid stream pressure at the nozzle exit and ambient pressure is substantially unchanged.

Some embodiments of the invention will now be de- ICC scribed by way of example with reference to the accompanying drawings of which:

FIGURE 1 is a general View of a ground mufller for reducing the noise of an aircraft engine while being run up.

FIGURE 2 is a longitudinal section of the ground mufller shown in FIGURE l. y

FIGURE 3 is a longitudinal section of another ground muier.

FIGURE 4 is a fragmentary detail of the ground muler shown in FIGURE 3.

FIGURE 5 is a longitudinal section of the upstream end of a modified form of the ground muler shown in FIGURE 2.

FIGURE 6 is an end view of the ground mutiler shown in FIGURE 5.

FIGURES 7 to 9 show diagrammatically three alternative shapes of ground muffler.

FIGURE 10 shows a reciprocating aircraft engine with a silencer in the exhaust pipe.

As shown in FIGURES 1 and 2 `the ground mutlier consists essentially of a divergent duct 1 which is internally of the form of a truncated right circular cone. The duct is mounted on a wheeled trolley 2 with its axis horizontal and is arranged so that, as shown in FIGURE l, its smaller end can be pushed up against 4the jet nozzle 3 of an aircraft 4 powered by a gas turbine jet propulsion engine, the smaller end of the duct accordingly constituting an inlet for receiving the jet stream discharged from the nozzle 3.

The duct 1 comprises four axially successive annular tapered wall sections 5, 6, 7, 8, the upstream section 5 having an inlet portion 5a tapered in an opposite sense to constitute a socket in which the jet nozzle 3 engages. Clamped between these wall sections are three axially spaced walls 9, 10, 11 extending transversely across the duct, the upstream wall 9 being spaced from the duct inlet, the wall sections 5-8 and the transverse walls 911 are all made of a refractory material such as a ceramic which is resistant to the hot jet stream. The wall sections 5-8 are substantially impervious but the transverse walls 9, 10 and 11 are formed with apertures to permit the flow of the jet stream through the duct. Thus the walls 9, 10 are made of a porous gas-pervious material while the wall 11 is formed with a large number of discrete perforations. Each wall presents a restricted flow area for the stream and constitutes a resistance to the flow, and accordingly a pressure drop in the stream exists across it.

The wall sections 5-8 are enclosed in an outer casing 12 spaced therefrom, the intervening space being packed with a sound absorbing material 13.

As the jet stream flows along the divergent duct, its velocity is reduced and its pressure is increased, and as is well known, the adverse pressure gradient tends to give rise to boundary layer separation from the duct wall. This must be avoided as far as possible as such separation gives rise to an increase in the jet noise. Accordingly the first transverse wall 9 is so located and produces such a pressure drop in relation to the local jet velocity and the configuration of the duct that boundary layer separation between the duct inlet and the wall 9 is substantially suppressed.

Each of the subsequent transverse walls '10, 11 is similarly so located and produces such a pressure drop that boundary layer separation in the space between that wall and the one immediately upstream thereof is substantially suppressed.

In a muter intended for use with a jet nozzle operating in an unchoked condition, i.e. discharging at ambient atmospheric pressure, the total pressure drop produced by all the walls is such that the static pressure at the duct inlet is maintained at atmospheric. Thus the muer exerts 'no Ybaclipressure on the engine and it is possible to run the engine up in exactly the same way as if the muienwere not there.

vIn `this embodiment, theI pressure drop, `across each transverse wall, 9, 1-0, 11 isof the order of the pressure rise',V dueto, the, deceleration of the stream in the spaeA immediately upstream of that Iwall i.e. in the space'be tween the wall 9` andthe duct inlet in the caseof the upstream wall 9, and in thez spaces betweenthe walls and lll, and 10` and 11 in the casesof thev walls 10 and-11 respectively. The spacing of each wall from thev wall upstream. thereof. and, inthe case ofthe upstream Wall* 9, yits spacing from the duct inlet depends upon they position at which boundaryA layer separation tendsvto` occur, the wall being located at or upstreamv pressure in the jet pipe and consequent choking of the,

engine, it is desirable that the llow area through thefrst wall 9,- shall ngt beless than the flow area of the inlet This requirement will limit the closeness to theduct. of the spacing of the Wall 9 from theduct inlet. Special means. may, therefore,` be provided. in order to v.further suppress any tendency for boundary layer sepa-ration to occur. Thus the duct is provided with so called vortexl generators between the duct inlet and the wall- 9. These comprise, projections 14v from the wall o f the. duct which areso arranged as to give rise to trailing vortices in the stream which have the effect of reducing the. tendency to boundary layer separation. Other forms of vortex generator may be used, or alternativelyprovision may be made for boundary layer suction as described below with reference to FIGURE 4, or for renewing the boundary layer` by blowing. If necessary, similar means to suppress boundary layer separation may4 also be provided in the spaces between the transverse Walls 9l andf'lll and 10 and 11.

If the muffler in intended for use with a jet stream discharged. from a choked nozzle, the engine will normally be operating under back pressure and it is desirable to `ensure that this back pressure is not increased to'any substantial extent. Thus the transverse walls will be such that they give rise to an overall pressure drop between the inlet and outlet of the duct equal tol the difference between theY jet exit and vambient pressures, so thatupressure at the duct inletis equal to the pressure l of( the `jetrstrearn when discharging freely.

By the Ause of a rnufiier as described,` above, the jet velocity may be reduced vto quite a low value in a very Sheri: 'axial distan@ with,` a. @responding reduction in jet noise. duct/may berapidly'divergent, of, say, 45 included angle.

The divergent duct is not necessarily straight, or of circular cross-section o r of constantdivergence along itsy length.V FIGURE 3 shows another form of ground` muier in which the divergence of the duct wall at rst increasesfrom the inlet andi then decreases, the downstream portion beinguntapered. In this embodiment, the, duct .'Wall consists ofa tubulardwall member 21 made of a heat resisting alloy such" as those commonly used in a gas turbine plant. In some cases this wall may be perforated. As inthe embodiment of FIGURE Z, the wallrnernber 21 is` enclosed by an outer casing 212, the intervening space being filled with sound absorbing rnaterialtZS'. Extending aeross the duct arefour transverse walls, 24; Z5,` 26, Z7; the upsteam wall-24 being spaced downstream from the duct inlet and the downstream wall '27 being at the upstream end of the untapered part of the duct. As shown in FIGURE 4, the transverse walls are of honeycomb construction, yconsisting of alternate flat and corrugated stripsJZS, 29 of heat resistant alloy afordingtherebetween passages for the ow of the jetvstrearn. As `in the embodiment vot FIGURE 2, these walls constitute resistances to the ilow and a pressure drop in the stream exists across` each ot them. The walls have such pressure drops and are so located-that the stream is decerlerated in the manner already described without substantial separation ot the boundary layer from the duct wall with a consequent reduction in ist noise.

Asinthe embodiment of FIGURE 2, special means may be required Vto suppress any tendency forY boundary layer separation to occur between theduct inlet and theV first transverse wall 24. Accordingly the duct wall includes arsection 21a between the duct inlet and wall 24.1madeotv a gas-p ervious material. Behind this sectionis an annular gallery 30 connected by pipe 31 to al suction pump 32. Thus boundary layer suction may be applied to this section of the ductv to assist in suppressing boundary layer separation. f

Boundarylayer suction may be applied in a like manner to anyother part of the duct wall. Alternatively boundary layer separation can be suppressed by the use ogvortex generators as shown in FIGURE 2 or provision may, be made for renewing the boundary layer at intervalsby blowing in streams of air.

Longitudinal splitters made of sound-absorbing material in known mannerv may be provided in the downstreamuntapered part ofthe duct.

inthe embodiments described above, the jet nozzle 3 isaclose riity in theinlet 5a to the duct, and if necessary sealing means may b e provided for ensuring a gas-tight connection. In the embodiment of` FIGURES 5 and 6, then-inlet 5a is formed with three ribs 5b with which the Y jet nozzle 5k engages so that there is a gap around the nozzle whereby air can be drawn from atmosphere to flow along andcool the walls of the duct. The gap must be large enough to ensure that the pressureat the nozzle is maintained., equal to/atmospheric pressure.

Some furtheralternative congurations of the duct are shown in FIGURES 7, 8 andy 9. In FIGURE 7, the duct 4,0is of.,v progressively increasing divergence alongk its length, andis thus of the form of a trumpet; in FIGURE `8, the duct 41 is curved so that thejetistreamis nally discharged in an'upwards direction; and in FIGURE 9, the. duct 42. changes in section along its length from a circulaninletvto very long narrow iishtail outlet. In any case, the duct may include a downstream untapered part as in the embodiment of FIGURE 3.

The invention mayalso be applied to the silencing of the exhaust gasjstream of `an engine when in normal operation, e.g. in ilight, providedwit is not desired to use the stream tovproduce propulsive thrust. Thus FIGURE 10 showsganaircraft engine 50 of the conventional in-line piston. typemounted ina nacelle 51 and driving a pro-y p ellerpSZ, 4rIhee'ngine exhaust is discharged through an exhaust manifold 53 and exhaust pipe 54 in which is embodied a ysilencer consisting of. a divergent duct section 55 hayingwalls formed with apertures extending across it as describedabove. 'I'ihis silencer lwill in eiect be a scaled-down version ofI the ground muffler of FIGURE 2. By" appropriatel choice ofthe location of the walls. and the pressure drop across them in relation to the exhaust gas velocity and-the configuration of the duct (according tfthe principles explained in connection with FIGURE 2), it can be 4arranged that no additional back pressure is produced on the engine and that its performance is not impaired. i

The silencer could of course be any one of the congurations shown in vFIGURES 3, 7, 8 andU9. v

The invention could similarly be applied to silencing the exhaust stream of a turbo-prop engine in flight. It.Y

will be appreciated, however, that the use of the invention will involve the loss of the thrust of the exhaust stream which is usually reckoned on as giving part of the forward thrust of a turbo-prop engine.

Other special means may be provided in any of the embodiments described for assisting in suppressing boundary layer separation. Thus the duct wall may be provided with slots for boundary layer suction, or the boundary layer may be renewed by blowing streams of air over the duct wall through suitably located slots.

Provision may also be made for varying the pressure drop existing across one or more of the transverse walls, e.g. by varying the flow area therethrough. In this way it may be possible to use a single ground muiiier for a particular jet engine under varying conditions e.g. operating both under and over the choked condition, or for a number of diiferent types of jet engine. For the same reason provision may be made for varying the area of the muiher outlet.

The invention may also have application in jet propulsion engine test beds.

We claim:

1. Apparatus for reducing the noise emitted by a uid stream discharged from a nozzle, comprising a duct which is divergent for at least part of its length and having a peripheral bounding wall, and open smaller and larger ends constituting respectively an inlet and an outlet for the stream, the stream accordingly being decelerated as it ilows along the duct; a wall within said divergent part of the duct, extending transversely across the whole extent of the duct, and located downstream of the smaller end but upstream of the position at which boundary layer separation from the peripheral wall caused by the divergence of the duct between said inlet and said transverse wall is liable to occur; and at least one further wall within said divergent part of the duct, extending transversely across the whole extent of the duct, and located downstream of said first-mentioned wall but upstream of the position at which boundary layer separation from the peripheral wall caused by the divergence of the duct between said transverse Walls is liable to occur; each said transverse wall having formed therein apertures distributed over its whole extent and aligned with the direction of flow of the stream along the duct, the flow area afforded by the apertures in each transverse wall being less than the cross-sectional area of the duct in the plane of that wall, whereby the apertures afford a restricted passage for the stream through that wall and accordingly give rise to a pressure drop in the stream across that Wall, and the ilow area afforded by the apertures in the first-mentioned transverse wall being at least as great as the flow area of said inlet; and said peripheral wall being shaped so that it diverges smoothly between said inlet and said first-mentioned transverse wall and between said transverse walls.

2. A ground muffler for reducing the noise emitted by a jet stream discharged from a jet propulsion nozzle comprising an open-ended duct having a bounding peripheral wall, a smaller end adapted to engage with said nozzle and constituting an inlet for the stream and a larger end constituting an outlet to atmosphere for the stream, said duct being divergent from said smaller end for part of its length and cylindrical for the remainder of its length to said larger end; a wall within said divergent part of the duct, extending transversely across the whole extent of the duct, and located downstream of the smaller end but upstream of the position at which boundary layer separation from the peripheral wall caused by the divergence of the duct between said inlet and said transverse wall is liable to occur; and at least one further wall within said divergent part of the duct, extending transversely across the whole extent of the duct, and located downstream of said first-mentioned wall but up stream of the position at which boundary layer separation from the peripheral wall caused by the divergence of the duct between said transverse walls is liable to occur; each said transverse wall having formed therein apertures distributed over its whole extent and aligned with the direction of flow of the stream along the duct, the ow area afforded by the apertures in each transverse wall being less than the cross-sectional area of the duct in the plane of that wall, whereby the apertures afford a restricted passage for the stream through that wall and accordingly give rise to a pressure drop in the stream across that wall, and the iiow area afforded by the apertures in the first-mentioned transverse wall being at least as great as the flow area of said inlet; and said peripheral wall being shaped so that it diverges smoothly between said inlet and said rst-mentioned transverse wall and between said transverse walls.

3. A muffler for use with an exhaust of a jet type engine, said muler comprising an inner wall having an intake opening and an outlet opening, said inlet opening being small with respect to said outlet opening, said wall having an intermediate portion in the form of a truncated cone, at least two transverse walls extending across said intermediate portion, and spaced from each other, each of said walls being of honeycomb-like form defining passages therethrough, a layer of sound absorbing material surrounding said inner wall and an outer Wall surrounding said sound absorbing material.

References Cited in the le of this patent UNITED STATES PATENTS 199,512 Buzzell Ian. 22, 1878 618,479 Pennington Ian. 31, 1899 1,685,701 Blanchard Sept. 25, 1928 1,813,189 Moore July 7, 1931 1,887,305 Helmore et al. Nov. 8, 1932 2,047,378 Martin July 14, 1936 2,229,576 Kirby Ian. 21, 1941 2,395,005 Kuhn Feb. 19, 1946 2,396,952 Huber Mar. 19, 1946 2,473,103 Lathers June 14, 1949 2,650,752 Hoadley Sept. 1, 1953 2,674,335 Lemmerman Apr. 6, 1954 FOREIGN PATENTS 638,954 Great Britain June 21, 1950 

